1. Field of the Invention
The present invention relates to a method of testing a withstand-voltage of a device utilizing a core conductor coated for insulation and a withstand-voltage testing apparatus employed in this method.
2. Description of the Related Art
FIGS. 1 to 3 show a conventional high withstand-voltage testing apparatus for testing a high withstand-voltage of a device 1 in which a core conductor 2 is coated with an insulating member 3 made of resin, or the like. For the test, the device 1 is enclosed by a grounded conductor 4. A three-pole discharge electrode 5 is constituted by a positive pole 5a, a negative pole 5b and a floating pole 5c. The positive pole 5a is connected to the core conductor 2, while the negative pole 5b is connected to the grounded conductor 4. A high-voltage pulse generator 6 is constituted by a high-voltage generator 6a and an oscillator 6b. A resistor R for detecting leakage current of the device 1 is inserted between the grounded conductor 4 and earth. A voltmeter 7 is provided for detecting voltage of the resistor R.
As shown in FIG. 3, in the case where a high-voltage cable 9 coupled with a spark plug 8 for automobiles is employed as the device 1, a resinous coating 10 of the high-voltage cable 9 has joints and pin holes. Thus, a high withstand-voltage test is performed mainly at coupling portions 10a and 10b of the high-voltage cable 9, where leakage current is likely to be produced. Accordingly, a distal end 8' of the high-voltage cable 9 adjacent to the spark plug 8 is supported by a coupling jig 11c.
Meanwhile, after a conductor 12 extending from the coupling portion 10a has been positively insulated by resin 13, or the like, the coupling portions 10a and 10b are supported by grounding jigs 11a and 11b, respectively and further are, respectively, surrounded by metallic beads 4a and 4b strung on threads and acting as the grounded conductor 4 so as to be grounded.
In the above described arrangement of the known high withstand-voltage testing apparatus, high-voltage pulses are applied to the core conductor 2 and the positive pole 5a of the discharge electrode 5 as shown in FIG. 1. When there is no leakage current in the work 1, pulsed discharge beams are continuously generated in a gap of the discharge electrode 5. On the contrary, when there is leakage current in the device 1, discharge does not take place at the discharge electrode 5 and electric current flows through the resistor R. In this case, by reading voltage of the voltmeter 7, it is determined that the work 1 has an improper withstand-voltage. It is to be noted that the voltmeter 7 may be replaced by a neon lamp, or other voltage indicating device.
In the conventional high withstand-voltage testing apparatus referred to above, leakage current in the work 1 is detected by the resistor R or the neon lamp. The resistor R or the neon lamp is an electric detector and therefore, is affected by electrical noises. Accordingly, if electrical noises become extremely large, it is necessary to raise the impedance between the detection system and the decision system. As a result, the decision system is susceptible to electrical noises and thus, functions unstably.
For example, in the conventional apparatus, even in the case where the device 1 has proper withstand-voltage and discharge is generated at the discharge electrode 5, leakage current due to moisture, etc. is detected at the device 1, thereby resulting in an erroneous determination in the high withstand-voltage test. Furthermore, such high withstand-voltage testing apparatus is disadvantageous in that an erroneous determination is likely to be made due to long response time from detection to decision or drop of detection sensitivity.